Recovery of sulfur dioxide from waste gases

ABSTRACT

A process is provided to efficiently and economically absorb and recover sulfur dioxide gas, and thus eliminate air pollution and produce a useful sulfur-containing product. Sulfur dioxide is recovered from a waste gas stream, such as the flue gas from a power boiler which burns sulfur-containing coal or other fuel, by scrubbing the waste gas stream with an aqueous solution of magnesium bisulfite-magnesium sulfite, Sulfur dioxide is absorbed into the aqueous scrubbing solution by reacting with magnesium sulfite to form magnesium bisulfite in solution. The resulting solution is divided into two portions. Magnesium hydroxide is added to the first portion at a controlled rate, to convert magnesium bisulfite to magnesium sulfite. The first portion is then recycled for further waste gas scrubbing. The magnesium sulfite and magnesium bisulfite content of the second solution portion are processed at elevated temperature to produce magnesium oxide and a gas stream of high sulfur dioxide content, usually about 10 percent by volume. The magnesium oxide is slaked with water to form magnesium hydroxide which is added to the first solution portion, and the gas stream of high sulfur dioxide content is utilized to produce a sulfur-containing product.

United States Patent [72] inventor lndravadan S. Shah Forest Hills, N.Y.21 Appl. No. 737,186 [22] Filed June 14, 1968 [45] Patented Nov. 2, 1971[73] Assignee Chemical Construction Corporation New York, N.Y.

[54] RECOVERY OF SULFUR DIOXIDE FROM WASTE GASES 6 Claims, 2 DrawingFigs.

[52] 11.8. CI 23/167, 23/2, 23/ 178 [51] int. Cl C01b 17/56,

COlb 17/72 [50] Field of Search 23/2, 129, 130, 131, 167,177, 178, 186

[56] References Cited UNITED STATES PATENTS 2,161,056 6/1939 Johnstoneet a1. 23/178 S 2,452,517 10/1948 Broughton 23/179 X 2,922,735 1/1960Johnstone 23/178 X 3,085,858 4/1963 Trubey et al. 23/130 3,475,12110/1969 Thornton 23/178 3,477,815 11/1969 Miller et a1. 23/178 23/13OX23/131X 3,273,961 9/1966 Rogersetal. 3,309,262 3/1967 Copeland etalABSTRACT: A process is provided to efficiently and economically absorband recover sulfur dioxide gas, and thus eliminate air pollution andproduce a useful sulfur-containing product. Sulfur dioxide is recoveredfrom a waste gas stream, such as the flue gas from a power boiler whichburns sulfurcontaining coal or other fuel, by scrubbing the waste gasstream with an aqueous solution of magnesium bisulfite-magnesiumsulfite, Sulfur dioxide is absorbed into the aqueous scrubbing solutionby reacting with magnesium sulfite to form magnesium bisulfite insolution. The resulting solution is divided into two portions. Magnesiumhydroxide is added to the first portion at a controlled rate, to convertmagnesium bisulfite to magnesium sulfite. The first portion is thenrecycled for further waste gas scrubbing. The magnesium sulfite andmagnesium bisulfite content of the second solution portion are processedat elevated temperature to produce magnesium oxide and a gas stream ofhigh sulfur dioxide content, usually about 10 percent by volume. Themagnesium oxide is slaked with water to form magnesium hydroxide whichis added to the first solution portion, and the gas stream of highsulfur dioxide content is utilized to produce a sulfur-containingproduct.

PATENTEDNUVZIQH 35 72 INDRAVADAN S. SHAH INVENTUR.

AGENT PATENTEU NUVZ I97! SHEET 2 OF 2 INDRAVADAN S. SHAH INVENTOR.

AGENT RECOVERY i SULFUR DIOXIDE FROM WASTE GASES BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates to the removaland recovery of sulfur dioxide from waste gas streams, in order toprevent air pollution and to recover a valuable sulfur-containingproduct. The invention is typically applicable to the waste flue gasgenerated by the combustion of a sulfur-containing fuel, such as theflue gas discharged by a coal-buming steam power plant. The invention isapplicable to the tail gas from a sulfuric acid production facility, inwhich case the recovered sulfur dioxide contained in the gas streamproduced by processing of the magnesium sulfite-bisulfite solution willgenerally be recycled to the sulfuric acid production facility, toproduce further sulfuric acid product. The process of the presentinvention may also be applied to the processing of the flue gasgenerated by the combustion of liquor, which is produced as a byproductin the magnesia base pulp digestion process.

2. Description of the Prior Art At present, the flue gases leaving apower plant or sulfuric acid plant stack are a major source of airpollution due to the presence of sulfur dioxide. The processing of wastegas streams to remove and recover sulfur dioxide is described in U.S.Pat. Nos. 1,212,199; 2,086,379, and 2,090,142. Disclosures relative toprocessing involving sulfite solutions include U.S. Pat. Nos. 2,210,405;2,375,786; 2,413,321 and 3,085,858. Technology relative to theprocessing of sulfite solutions in wood pulp manufacture is disclosed inU.S. Pat. Nos. 716,330; 830,996; 1,097,781; 1,378,617; 1,499,898;1,549,189; 1,637,353; 1,828,690; 2,042,477; 2,042,478; 2,047,627;2,141,886; 2,147,161; 2,147,162; 2,190,612; 2,351 ,78); 2,572,929;2,637,627; 2,872,289; and 3,273,961.

SUMMARY OF INVENTION In the present invention, sulfur dioxide isrecovered from waste gases and then regenerated in a concentrated form,so that valuable products such as liquid sulfur dioxide, sulfuric acid,elemental sulfur, etc., are produced. The process of the presentinvention removes up to 99percent or more of the sulfur dioxide fromflue gas or other waste gas by absorption. Fly

ash is also simultaneously scrubbed from the flue gas. The flue gasleaving the absorption system and discharged to the atmosphere isessentially free of sulfur dioxide and fly ash, and does not cause anair pollution problem. The absorbing liquor consists of an aqueoussolution of magnesium sultite which may also contain magnesiumbisulfite. The absorption reaction takes place between magnesiumsulfite, sulfur dioxide and water to form magnesium bisulfite in aqueoussolution. The quantity of magnesium sulfite in the scrubbing andabsorbing liquor is maintained in excess of the theoretical requirementto absorb essentially all of the sulfur dioxide.

The liquor derived from the scrubbing and absorption step now containsmagnesium bisulfite, residual magnesium sulflte and fly ash. The liquormay also contain a small proportion of magnesium sulfate, in instanceswhen sulfur trioxide is present in the original waste gas. The liquor ispassed through a filter or centrifuge, where fly ash isremoved from thesolution. A major portion of the clear solution from the filter isreturned to the absorption system, and the remaining solution preferablyenters a reaction tank. Magnesium oxide or 'magnesium hydroxide is addedto the solution in the reaction tank, and reacts with magnesiumbisulfite to form further magnesium sulfite. Magnesium oxide will alwaysbe added in excess, in order to insure that all magnesium bisulfite isconverted to magnesium white. The magnesium sulfite has very limitedsolubility and will precipitate out of solution as solid crystals. Themagnesium sulfate which may be formed will also precipitate out beyondthe solubility limits. The resulting slurry is then sent through afilter or centrifuge, in which solid precipitate is separated fromresidual liquor.

The residual liquor is then returned to the absorption system, andcombined with the major portion of the clear solution from the fly ashfilter. Magnesium hydroxide is added to the combined solution, andreacts with the magnesium bisulfite derivedfrom the clear solution toform further magnesium sulflte. The addition of magnesium hydroxide iscontrolled to maintain a desired magnesium sulfite concentration in theresulting solution, which is returned to the absorption system forfurther waste gas scrubbing.

The solid precipitate of magnesium sulfite crystals separated from theresidual solution is then calcined, using either direct or indirectcalcination. The magnesium sulfite decomposes at elevated temperature toyield solid magnesium oxide and sulfur dioxide gas. The resultingconcentrated gas stream containing sulfur dioxide, which is dischargedfrom the calciner, usually contains at least 10percent sulfur dioxidecontent by volume, and is utilized to make a suitable sulfurcontainingproduct, such as liquid sulfur dioxide, sulfuric acid, or elementalsulfur.

The solid magnesium oxide is divided into two streams. A first stream issent to the reaction tank for conversion of magnesium bisulfite tosulfite, and the balance of the magnesium oxide is slaked with water toform magnesium hydroxide, which is added to the combined solution passedto the absorption system.

In summary, the process includes:

1. Recovery of sulfur dioxide from a waste gas stream, such as the fluegas from a power boiler which burns sulfur containing coal or otherfuel, by scrubbing the waste gas stream with an aqueous solution ofmagnesium bisulfite-magnesium sulfite. The sulfur dioxide is absorbed byreacting with the active chemical component, namely magnesium sulflte,inaqueous solution, to form magnesium bisulfite. 2. The resultingsolution is divided into two portions. One portion is the recyclestream, and the other portion the bleed stream. Magnesium hydroxide isadded to the recycle stream, and reacts with magnesium bisulfite to formmagnesium sulfite. 3. An excess of magnesium oxide is added to the bleedstream, so that magnesium sulfite is formed and precipitated in thecrystal form. 4. The precipitated magnesium sulfite and unreactedmagnesium oxide are separated from the mother liquor, which is returnedto the absorption system. 5. The mixture of magnesium sulfite andmagnesium oxide are then calcined, to form solid magnesium oxide and agas stream of high sulfur dioxide content, usually above 10percent byvolume. 6. Some of the magnesium oxide is returned to the reaction tank,to precipitate magnesium sulfite as described in (3) supra. Theremaining magnesium oxide is slaked with water to form magnesiumhydroxide, which is added to the recycle stream according to (2) supra.7. The gas stream of high sulfur dioxide content is utilized to producea sulfur-containing product,

such as sulfuric acid, elemental sulfur, etc. For the production ofelemental sulfur, hydrogen or a gaseous hydrocarbon may be employed indirect reduction of sulfur dioxide to sulfur, or a portion of the sulfurdioxide may be converted to hydrogen sulfide by reaction with hydrogenor a gaseous hydrocarbon, with the hydrogen sulfide then being reactedwith the balance of the sulfur dioxide in accordance with the Clausprocess, to form elemental sulfur.

In an alternative embodiment of the invention, a portion or all of theclear solution from the fly ash filter is concentrated by evaporation,and the resulting concentrated solution or slurry of magnesiumbisulfite-sulfite is passed to a fluidized bed reactor. A fluid bed ofsolid magnesium oxide particles is maintained in ebullient or fluidmotion and is heated by the injection of a fluid hydrocarbon fuel andheated air below or into the bed. The water content of the concentratedsolution is completely evaporated in the fluid bed, and the solid saltsare decomposed to yield further magnesium oxide and sulfur dioxide. Agas stream rich in sulfur dioxide is withdrawn from the top of the fluidbed and processed for sulfur recovery, while a side stream of solidmagnesium oxide particles is withdrawn from the fluid bed and slakedwith water to form magnesium hydroxide, which is added to the solutionbeing recycled to the waste gas scrubbing an absorption system.

The system of the present invention provides several importantadvantages. Essentially all of the sulfur dioxide is removed from thewaste gas stream. When the waste gas is a flue gas, entrained solidssuch as fly ash are also removed. Thus, air pollution is curtailed andprevented. The chemicals cost for the process is essentially negligible,since all of the magnesium oxide is recovered. The sulfur dioxide isregenerated in a concentrated form, suitable for the preparation ofliquid sulfur dioxide, sulfuric acid, or any other suitable product.

It is an object of the present invention to provide an improved processfor the removal of sulfur dioxide from waste gas stream.

Another object is to provide a process for the removal of sulfur dioxideand fly ash from flue gas stream.

A further object is to provide a process for recovering sulfur dioxidein concentrated form from waste gases containing a small proportion ofsulfur dioxide.

An additional object is to prevent the pollution of the atmosphere bywaste gases containing sulfur dioxide.

Still another object is to provide a process for recovering sulfurdioxide from waste gas streams which has a negligible cost andrequirement for chemicals for the process.

Still a further object is to provide an improved process for scrubbingsulfur dioxide from waste gas streams using an aqueous absorbentsolution containing magnesium sulfite.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPT ION OF THE DRAWINGS AND PREFERRED EMBODIMENTS Referring now tothe drawings,

FIG. 1 is a flowsheet of one embodiment of the invention, in whichmagnesium sulfite is precipitated and calcined for decomposition, and

FIG. 2 is a flowsheet of an alternative embodiment of the invention, inwhich a portion of the scrubbing solution is concentrated by evaporationand passed to a fluidized bed reactor for decomposition.

Referring now to FIG. 1, a fuel stream 1 and combustion airstream 2 arepassed into power boiler 3, which is typically a steam boiler in whichthe fuel stream 1 is burned to generate heat and vaporize water to formusable high pressure steam. Stream 1 may consist of coal, Bunker Cresidual oil, or any other suitable fuel which contains combustiblecomponents and sulfurous impurity. The combustion of fuel stream 1 withairstream 2 in unit 3 serves to generate a flue gas stream 4, whichcontains sulfur dioxide, fly ash, water vapor, residual free oxygen andinerts including carbon dioxide and nitrogen. Stream 4 is processed inaccordance with the present invention, to produce a final gas stream ofnegligible sulfur dioxide and fly ash content which is suitable fordischarge to the atmosphere without causing air pollution.

Stream 4 is scrubbed with an aqueous magnesium sulflte solution, whichmay also contain magnesium bisulfite and which absorbs sulfur dioxideand entrains fly ash particles from stream 4. The scrubbing of stream 4is preferably attained in a venturi contactor, which attains uniformdispersion of the liquid phase into the gas phase and does not clog orplug due to fly ash deposition or buildup. Stream 4 is preferably passedcentrally downwards through the top of venturi contactor-scrubber 5, andis accelerated to a high velocity by the downwardly converging inclinedsidewalls 6. In instances when unit 5 is cylindrical, elements 6 will bein the form of a funnel or inverted truncated cone. The aqueousscrubbing and absorbent solution stream 7, which contains dissolvedmagnesium sulfite and may also contain dissolved magnesium bisulfite andentrained solid particles of magnesium sulfite, is admitted into theupper part of unit 5 adjacent to the upper terminus of each element 6.Stream 7 is at an initial temperature typically in the range of about 30to 90 C., and typically contains about 0.5 to 2 percent magnesiumsulfite content by weight. Stream 7 flows downwards across the uppersurface of element 6, and in some instances stream 7 may be passedtangentially to the inner surface of unit 5, so as to flow downwardsacross element 6 with a whirling circular motion. In any case, stream 7is projected inwards and transversely into the highly accelerated gasstream at the lower end of element 6, and the liquid phase iseffectively and uniformly dispersed into the gas stream in the form ofliquid droplets, with resultant rapid attainment of equilibrium betweenthe phases. Consequently, absorption of sulfur dioxide into the liquidphase, formation of magnesium bisulfite in the liquid phase andentrainment of fly ash rapidly take place in unit 5. The scrubbed gasphase, now substantially free of sulfur dioxide and fly ash, separatesfrom the liquid phase in the lower portion of unit 5 and is dischargedto atmosphere via stream 8, which may extend to a flue gas disposalstack, not shown.

The liquid phase collected in the bottom of unit 5 now consists of anaqueous solution or slurry which contains residual magnesium sulfite,magnesium bisulfite and fly ash. In in stances when stream 4 contains asmall proportion of sulfur trioxide, the liquid phase in the bottom ofunit 5 will also contain magnesium sulfate. A liquid stream 9 iswithdrawn from the bottom of unit 5, and stream 9 is passed to filter orcentrifuge unit 10, in which solid fly ash is separated from the liquidphase and discharged via stream 11. The clear solution stream 12discharged from unit 10 is divided into stream 13, which is generally amajor portion of stream 12 and is recycled as will be described infra,and stream 14 which is further processed to precipitate solid magnesiumsulfite.

Stream 14 is passed into reaction tank 15, together with magnesium oxidestream 16, which is derived from within the process as will appearinfra. Stream l6 reacts in unit 15 with magnesium bisulfite derived fromstream 14, to form further magnesium sulfite. Sufficient magnesium oxideis added via stream 16 to react with all of the magnesium bisulfite, andexcess magnesium sulfite precipitates from the solution in unit 15 inthe form of solid magnesium sulfite crystals. The resultant slurry ofsolid magnesium sulfite crystals in saturated magnesium sulfite solutionis discharged from unit 15 via stream 17, which is passed to filter orcentrifuge unit 18 in which the solid magnesium sulfite phase isseparated from the liquid solution phase. The resulting liquid solutionphase stream 19 is recycled to absorption as will appear infra, whilethe solid magnesium sulfite crystals stream 20 is processed at elevatedtemperature to form a gas stream rich in sulfur dioxide, and solidmagnesium oxide.

Stream 20 is passed into calciner or rotary kiln 21, which may beindirectly heated or more typically direct-fired, with fluid hydrocarbonfuel stream 22 and combustion air stream 23 being admitted into thesolids discharge end of unit 21. The combustion of stream 22 in unit 21serves to generate a highly elevated temperature generally in the rangeof 700 to l,000 C. within unit 21, and the magnesium sulfite feed stream20 is decomposed within unit 21 to yield sulfur dioxide gas and solidmagnesium oxide. The resulting sulfur dioxide-rich gas 24 dischargedfrom unit 21 is passed to further processing, not shown, for thepreparation of liquid sulfur dioxide, sulfuric acid, elemental sulfur,or other sulfur-containing product. Stream 24 will contain at least 5percent by volume of sulfur dioxide content, and usually contains morethan 10 percent sulfur dioxide content by volume.

The solid magnesium oxide stream 25 discharged from unit 21 is nowdivided into stream 16, which is recycled to unit 15 as described supra,and stream 26, which is passed to slaker or slaking system 27, in whichwater stream 28 is employed to slake the magnesium oxide and formmagnesium hydroxide. The resulting magnesium hydroxide stream 29withdrawn from unit 27 is combined with streams-13 and 19 to form stream7. The magnesium hydroxide stream 29 serves to convert magnesiumbisulfite in stream 13 into magnesium sulfate.

Referring now to FIG. 2, an alternative procedure within the scope ofthe present invention is illustrated. The initial stages of the processin FIG. 2 are similar to FIG. 1, and therefore these stages will only bebriefly described. Sulfur-containing fuel stream and combustionairstream 31 are reacted in combustion unit 32, which is typically asteam power boiler. The generated flue gas stream 33 contains sulfurdioxide and entrained fly ash which are removed in accordance with thepresent invention. Stream 33 is passed into the top of gas scrubbing andabsorption unit 34, and is accelerated by converging venturi baffles 35.Scrubbing liquid stream 36, which consists of an aqueous solutioncontaining magnesium su'lfite, is passed downwards on the upper surfaceof units 35 and is projected into the-highly accelerated gas stream atthe lower ends of sections 35. The gas stream is thus scrubbed withdroplets of the liquid solution, in unit 34, and the resultant scrubbedgas stream, now substantially devoid of sulfur dioxide and fly ash, isdischarged from unit 34 via stream 37.

A liquid solution or slurry stream 38 is removed from the bottom of unit34. Stream 38 contains residual magnesium sulflte, magnesium bisulfiteformed by the reaction of absorbed sulfur dioxide with magnesiumsulfite, and entrained solid fly ash. Stream 38 is divided into stream39, which is recycled to absorption as will appear infra, and stream 40,which is further processed to recover magnesium oxide values and a gasstream rich in sulfur dioxide. Stream 40 is passed through solids filteror centrifuge unit 41, to remove solid fly ash which is discharged viastream 42. The clear liquid solution stream 43 discharged from unit 41now contains magnesium sulfite and magnesium bisulfite in aqueoussolution.

Stream 43 is now sprayed or otherwise passed into evaporator 44, forevaporation of a portion of the liquid water from the solution by directcontact with hot airstream 45, which is admitted into the lower portionof unit 44 and flows upwards countercurrent to the falling liquiddroplets. Cooled airstream 46 of high water vapor content is dischargedfrom the upper part of unit 44 to atmosphere.

A highly concentrated aqueous solution or slurry collects in the bottomof unit 44, and is removed via stream 47, which passes into fluidizedbed reactor48. A hot fluidized bed consisting principally of magnesiumoxide particles is maintained in turbulent or ebullient motion withinunit 48, by injecting a fluid hydrocarbon fuel stream 49 and preheatedairstream 97 50 into the bottom or lowerportion of the unit 48. Thecombustion of stream 49 with stream 50 within unit 48 serves to maintainthe fluidized bed in unit 48 at a highly elevated temperature, generallyin the range of about 700 to l,000 C. Stream 47 is dispersed into thetop of the bed in unit 48, and the water content of stream 47 is rapidlyvaporized, with deposition of solid magnesium sulfite and magnesiumbisulfite in the upper part of the bed. The deposited magnesium saltsare decomposed in the bed due to the elevated temperature, withresultant addition of magnesium oxide to the fluid bed and evolution ofsulfur dioxide into the gas phase rising through the bed.

A hot gas stream 51 is withdrawn from the upper part of unit 48, andstream 51 contains sulfur dioxide and water vapor in addition to gaseouscombustion products derived from the reaction of streams 49 and 50.Stream 51 is cooled in heat exchanger 52, with concomitant condensationof water vapor to liquid water, by heat exchange with ambient airstream53, which is passed through the shell of unit 52 external to the heatexchange tubes and is thereby heated to a temperature generally in therange of 100 to 500 C. The resultanthot airstream 54 discharged from theshell of unit 52 is divided to form streams 50 and 54, which areutilized as described supra.

The process gas stream 55 removed from the bottom of unit 52 nowcontains entrained droplets of liquid water, which are removed bypassing stream 55 into gas-liquid separator 56, which is any suitablecyclonic or baffled apparatus for separating entrained liquid from agaseous stream. The water-free gas stream 57 discharged from unit 56 isnow rich in sulfur dioxide, and is utilized to produce any desiredsulfur-containing product, such as liquid sulfur dioxide, sulfuric acidor elemental sulfur, by any conventional procedure. Thus, if liquidsulfur dioxide is a desired product, stream 57 may be cooled byrefrigeration to selectively condense liquid sulfur dioxide. Whensulfuric acid is the desired product, stream 57 will generally be passedto a conventional sulfuric acid production facility which producessulfuric acid by catalytic oxidation of sulfur dioxide to sulfurtrioxide followed by absorption of the sulfur trioxide in concentratedsulfuric acid or oleum.

The liquid water phase separated from the gas stream in unit 56 isremoved via stream 58, which is preferably utilized within the processas slaking water in the magnesium oxide slaker or slaking system 59.Magnesium oxide stream 60 is passed fromthe lower portion of the fluidbed in unit 48 to slaker 59, and is slaked with water stream 58 to formmagnesium hydroxide. Additional makeup water may be added to unit 59 asrequired via stream 61. The resulting magnesium hydroxide stream 62withdrawn from unit 59 is combined with stream 39 to form stream 36. Theaddition of stream 62 to stream 39 serves to convert magnesium bisulfitein stream 39 to'magnesium sulfite.

Numerous alternatives within the scope of the present invention willoccur to those skilled in the art. The ranges of process variables suchas temperature enumerated supra constitute preferred embodiments foroptimum utilization of the process concepts of the invention, and theprocess of the present invention may be practiced outside of theseranges in suitable instances. Units 3 or 32 may typically consist of anytype of facility or process which generates a waste gas streamcontaining sulfur dioxide. Thus, the invention is applicable to sulfuricacid production facilities, which generate a tail gas stream containingsulfur dioxide. In this case, streams 4m 33- could consist of a tail gasdischarged from the sulfur trioxide absorption unit, and the tail gascontaining sulfur dioxide would be free of entrained solid particles. Insuch instances, when the waste gas is free of entrained solids, units 10or 41 would be omitted. lf units 3 or 32 are sulfuric acid productionfacilities, the main feed streams to the facility will consist of asulfur-containing material such as elemental sulfur, pyrites or othersulfide ore, sludge acid from petroleum refining, hydrogen sulfide,etc., together with process air, and in this case the final'process gasstream 24 or 57 which is rich in sulfur dioxide may be passed to therespective sulfuric acid production facility 3 or 32. Units 5 and 34 mayconsist of any suitable device for attaining intimate contact scrubbingof a gas stream witha liquid absorbent solution, and in some cases aplurality of units or stages may beprovided in series, withcountercurrent flow of the gas and liquid streams through the stages.Unit 15 will generally be provided with a suitable internal agitationand mixing device such as a rotary stirrer, not shown. The calciner 21may be any suitable elevated temperature solids heater, and in somecases a fluidized bed unit similar to reactor 48 may be provided insteadof calciner 21. Referring to FIG. 2, unit 44 may consist of any suitableliquid evaporator, and in some instances unit 44 will consist of afalling film evaporator provided with vertical internal tubes, with theliquid stream 43 flowing down the inner surface of the tubes as a thinliquid film and hot airstream 45 flowing upwards through the tubes. inthis case, the tubes will generally be externally heated by highpressure steam or other suitable heating fluid. Finally, in some casesunit 48 may be replaced by a suitable rotary kiln or the like, or anysuitable apparatus unitfor dehydrating and roasting a slurry.

An example'of application of the process of the present invention to atypical sulfur dioxide-containing waste gas stream derived from acommercial facility-sulfuric acid production will now be described.

EXAMPLE The process of the present invention as shown in FIG. 1 wasapplied to the treatment of the waste tail gas stream from a commercialsulfuric acid production facility. Following are the flow rates ofcomponents in the major process streams.

FLOW RATE OF COMPONENT, KGJMIN.

Nitrogen Magne- Stream plus Sulfur Net Mg slum Temp., No. oxygen dioxideWater as MgO sulfite C Nora-The ealelner 21 was operated at about 950 C.

lclaim:

l. A process for the recovery of sulfur dioxide from a waste gas streamcontaining sulfur dioxide which comprises scrubbing said waste gasstream with an aqueous solution free of solid magnesium sulfite andcontaining in the range of about 0.5 percent to 2.0 percent by weight ofdissolved magnesium sulfite, said aqueous solution being at an initialtemperature in the range of about 30 to 90 C., whereby sulfur dioxide isabsorbed from said waste gas stream into said aqueous solun'on andreacts with dissolved magnesium sulfite to form magnesium bisulfite,dividing the resulting aqueous solution containing magnesium bisulfiteand residual magnesium sulfite into a first portion and a secondportion, adding magnesium hydroxide to said first solution portion,whereby magnesium bisulfite is converted to magnesium sulfite in saidfirst solution portion, recycling the resulting first solution portionto scrubbing of said waste gas stream as said aqueous solution, addingmagnesium oxide to said second solution portion, whereby magnesiumbisulfite is converted to magnesium sulfite, separating precipitatedsolid magnesium sulfite from the resulting solution, adding theresulting solution of low magnesium sulfite content to said firstsolution portion, calcining said solid magnesium sulfite at elevatedtemperature to produce magnesium oxide and a gas stream of high sulfurdioxide content, slaking a portion of said magnesium oxide with water toproduce said magnesium hydroxide, recycling the balance of saidmagnesium oxide by addition to said second solution portion, andpreparing a sulfurcontaining product from said gas stream of high sulfurdioxide content.

2. The process of claim 1, in which said waste gas stream is the tailgas from the sulfuric acid production facility, and said gas stream ofhigh sulfur dioxide content is passed to said sulfuric acid productionfacility and utilized to produce sulfuric acid.

37 The process of claim 1, in which said waste gas stream is a flue gasderived from the combustion of a sulfur-containing fuel, said flue gascontaining solid fly ash which is entrained in said resulting aqueousmagnesium sulfite-bisulfite solution, and said entrained solid fly ashis removed from said resulting aqueous solution prior to dividing saidresulting aqueous solution into a first portion and a second portion.

4. A process for the recovery of sulfur dioxide from a waste gas streamcontaining sulfur dioxide which comprises scrubbing said waste gasstream with an aqueous solution free of solid magnesium sulfite andcontaining in the range of about 0.5 to 2.0 percent by weight ofdissolved magnesium sulfite, said aqueous solution being at an initialtemperature in the range of about 30 to C., whereby sulfur dioxide isabsorbed from said waste gas stream into said aqueous solution andreacts with dissolved magnesium sulfite to form magnesium bisulfite,dividing the resulting aqueous solution containing magnesium bisulfiteand residual magnesium sulfite into a first portion and a secondportion, adding magnesium hydroxide to said first solution portion,whereby magnesium bisulfite is converted to magnesium sulfite in saidfirst solution portion, recycling the resulting first solution portionto scrubbing of said waste gas stream as said aqueous solution,concentrating said second solution portion by evaporation in directcontact with a first hot airstream, passing the concentrated secondsolution portionto a fluidized be of solid magnesium oxide particles inwhich a fluid hydrocarbon fuel is burned with a second hot airstream,withdrawing magnesium oxide from the lower portion of said bed, slakingsaid withdrawn magnesium oxide with water to produce said magnesiumhydroxide, withdrawing a hot flue gas stream containing water vapor anda high proportion of sulfur dioxide from the top of said bed, coolingsaid hot gas stream by indirect heat exchange with a third airstream,whereby said third airstream is heated and water vapor is condensed fromthe cooled gas, separating condensed liquid water from the cooled gasstream of high sulfur dioxide content, and dividing the resulting heatedthird airstream to form said first hot airstream and said second hotairstream.

5. The process of claim 4, in which said waste gas stream is the tailgas from a sulfuric acid production facility, and said gas stream ofhigh sulfur dioxide content is passed to said sulfuric acid productionfacility and utilized to produce sulfuric acid.

6. The process of claim 4, in which said waste gas stream is a flue gasderived from the combustion of a sulfur-containing fuel, said flue gascontaining solid fly ash which is entrained in said resulting aqueousmagnesium sulfite-bisulfite solution, and said entrained solid fly ashis removed from said resulting aqueous solution prior to dividing saidresulting aqueous solution into a first portion and a second portion.

lnlnltn mas

2. The process of claim 1, in which said waste gas stream is the tailgas from the sulfuric acid production facility, and said gas stream ofhigh sulfur dioxide content is passed to said sulfuric acid productionfacility and utilized to produce sulfuric acid.
 3. The process of claim1, in which said waste gas stream is a flue gas derived from thecombustion of a sulfur-containing fuel, said flue gas containing solidfly ash which is entrained in said resulting aqueous magnesiumsulfite-bisulfite solution, and said entrained solid fly ash is removedfrom said resulting aqueous solution prior to dividing said resultingaqueous solution into a first portion and a second portion.
 4. A processfor the recovery of sulfur dioxide from a waste gas stream containingsulfur dioxide which comprises scrubbing said waste gas stream with anaqueous solution free of solid magnesium sulfite and containing in therange of about 0.5 to 2.0 percent by weight of dissolved magnesiumsulfite, said aqueous solution being at an initial temperature in therange of about 30* to 90* C., whereby sulfur dioxide is absorbed fromsaid waste gas stream into said aqueous solution and reacts withdissolved magnesium sulfite to form magnesium bisulfite, dividing theresulting aqueous solution containing magnesium bisulfite and residualmagnesium sulfite into a first portion and a second portion, addingmagnesium hydroxide to said first solution portion, whereby magnesiumbisulfite is converted to magnesium sulfite in said first solutionportion, recycling the resulting first solution portion to scrubbing ofsaid waste gas stream as said aqueous solution, concentrating saidsecond solution portion by evaporation in direct contact with a firsthot airstream, passing the concentrated second solution portion to afluidized bed of solid magnesium oxide particles in which a fluidhydrocarbon fuel is burned with a second hot airstream, withdrawingmagnesium oxide from the lower portion of said bed, slaking saidwithdrawn magnesium oxide with water to produce said magnesiumhydroxide, withdrawing a hot flue gas stream containing water vapor anda high proportion of sulfur dioxide from the top of said bed, coolingsaid hot gas stream by indirect heat exchange with a third airstream,whereby said third airstream is heated and water vapor is condensed fromthe cooled gas, separating condensed liquid water from the cooled gasstream of high sulfur dioxide content, and dividing the resulting heatedthird airstream to form said first hot airstream and said second hotairstream.
 5. The process of claim 4, in which said waste gas stream isthe tail gas from a sulfuric acid production facility, and said gasstream of hiGh sulfur dioxide content is passed to said sulfuric acidproduction facility and utilized to produce sulfuric acid.
 6. Theprocess of claim 4, in which said waste gas stream is a flue gas derivedfrom the combustion of a sulfur-containing fuel, said flue gascontaining solid fly ash which is entrained in said resulting aqueousmagnesium sulfite-bisulfite solution, and said entrained solid fly ashis removed from said resulting aqueous solution prior to dividing saidresulting aqueous solution into a first portion and a second portion.